Patch MiniSandbox to support i686 platform and improve integrity

This patch proposes to fix the following issues with rayage.MiniSandbox,

1. syscall number of open() was hard-coded as 2, which is correct on x86_64 but incorrect for i686;
2. use of ptrace.debugger.PtraceProcess was unnecessary if the sole purpose was to extract the path argument of open(); use Sandbox.dump(T_STRING, address) instead; moreover, attaching to the sandboxed process may change process hierarchy and potentially invalidate some internal states of the sandbox;